Warp stability of lumber and wood products is an increasingly important consideration. Three types of warp, known as crook, bow, and cup, can be traced to differential length change within a board. FIG. 2 of Perstorper et al., Quality of timber products from Norway spruce, WOOD SCI. TECH. 29 (1995), 339-352, incorporated by reference herein, illustrates different types of warp. Crook refers to in-plane, facewise curvature of wood relative to a longitudinal axis. Bow refers to in-plane facewise curvature relative to a longitudinal axis. Crook and bow are closely related and differ primarily according to the planar surface used to define the warp. Crook refers to in-plane, facewise curvature of wood relative to a length axis. Twist, another type of warp, refers to a rotational instability about an axis of wood (usually the longitudinal axis). Twist appears to be associated with varying grain angle patterns (Brazier). Warp tendency apparently is influenced by a myriad of factors (see Table 1).
TABLE 1 Factor Reference Authors Compression wood Ying, Kretschmann, Bendtsen Drying stresses Martensson and Svensson Earlywood vs. late wood Kifetew, Lindberg, Wiklund; Pentoney grain angle Balodis, Ormarsson log sweep Taylor and Wagner Longitudinal shrinkage Ormarsson; Simpson and Gerhardt; Ying, Kretschmann, Bendtsen: McAlister and Clark Microfibril angle Barber and Meylan; Tang and Smith; Ying, Kretschmann, Bendtsen; Walker Moisture content gradients Simpson and Gerhardt radial and tangential Kifetew, Lindberg, Wiklund; Meylan shrinkages Specific gravity Pentoney; Ying, Kretschmann, Bendtsen stress and strain Ormarsson; Sandland; Hsu and Tang; Fridley and Tang; Simpson and Gerhardt, Iridayaraj and Haghighi
Dimensional and warp stability have always been valued attributes. Furthermore, new products emerging from dimension lumber, such as premium-grade joists and studs, require superior dimensional and warp stability performance. The ability to quantify warp potential of wood products would enhance the capability of the forest products industry to service these important markets.
Moreover, inefficient processing of raw timber and lumber wastes tremendous forest resources. Lumber warp reduces product grade and product value. Additionally, warp-prone lumber and lumber products perform poorly in uses or environments unsuitable for warp-prone wood. Millions of dollars are wasted every year because no method exists for efficiently and accurately detecting warp-prone lumber.
If warp-prone wood could be nondestructively identified during or prior to processing and product placement, processing raw timber and lumber into wood products would become more efficient. Raw logs could be culled prior to manufacturing, and wood-products manufacturing processes could be altered to direct raw lumber to various end products according to quality and value. For example, warp-prone trees could be identified while standing in forests or after cutting, and processed into products where warp is an irrelevant consideration (e.g. paper products, chipping, etc.). Green warp-prone lumber could be identified at the mill, separated, and kiln-dried using special warp-reducing techniques (e.g. rapid-drying, high-heat drying, final steaming, restraint-drying, etc.). Lumber having low warp potential could be dried using simpler and more economical methods.
Natural resources are unnecessarily wasted by using certain types of wood in inappropriate applications. If warp tendency of raw logs could be predicted, then warp-prone logs could be processed differently. For example, warp-prone logs could be cut into lumber with cuts being coordinated to reduce warp. The orientation of boards taken from certain logs could be altered to reduce warp, or the thickness of the lumber could be varied, since thicker lumber generally warps less. Alternatively, warp-prone logs could be culled and processed for specific uses (e.g. chipped, lumber for pallets, etc.). Lumber cut from warp-prone logs also could be specially processed (e.g. special kiln drying techniques) or used in selected applications (e.g. relative constant moisture applications).
Additionally, warp-prone lumber could be identified for use in only certain applications. For example, exterior window and door casings experience fluctuating moisture and temperature conditions during use. Warp prone lumber, even if initially straight when dried, could warp in such changing environments. Consequently, if warp-prone lumber could be identified, its use in warp-inducing environments could be avoided. Extremely warp-prone wood may be suitable only for uses where warping is not a significant problem (e.g. for pallets, landscape applications, etc.). In such cases, warp-prone green lumber could be processed without expensive drying techniques.
Warp stability has been studied from both the experimental and theoretical viewpoints. For example, earlier studies explored the links between drying warp and certain lumber characteristics, such as knots, slope-of-grain, and juvenile-wood content [Beard, J., et al., The influence of growth characteristics on warp in two structural grades of southern pine lumber, 43 FOREST PROD. J. 6, 51 (June 1993); Balodis, V., Influence of Grain Angle on Twist in Seasoned Boards, 5 WOOD SCIENCE 44-50 (1972)]. While some relationships were discovered, no commercially viable processes for detecting warp apparently have been developed.
Others have attempted to mathematically model the mechanical phenomena that govern warp instability. A general approach considers elastic, shrinkage, creep, and mechanosorptive elements, including their anisotropic variability and temperature dependence. Such models are complicated. See, e.g., Ormarsson (1995).
Matthews et al.'s U.S. Pat. No. 4,606,645, which is incorporated herein by reference, describes measuring fiber angle in a fibrous solid material relative to three mutually orthogonal reference axes. The '645 patent is understood to teach the measuring and analysis of light reflected from a wood sample to determine the grain angle of the sample. These measurements are then understood to be used in evaluating the strength of the wood. This reference is not understood to relate to determining warp potential of wood.
Kliger et al. teaches a destructive method for analyzing a board. Longitudinal shrinkage was determined by cutting sticks from a piece of lumber, averaging the shrinkage of each stick to determine a single value for longitudinal shrinkage, and modeling crook. Kliger teaches only a fairly approximate method for modeling crook. Kliger's method also depends on destroying the wood piece to determine crook. Furthermore, the authors employed a model which specified only a single radius of curvature whereas warp in wood can occur about more than one radius of curvature.
A practical and accurate method for predicting crook has, despite extensive efforts, not been developed. Additionally, the amount of information which must be known to predict crook has proved daunting.